1. Field of the Invention
The present invention relates generally to the field of preamble formats for multi input multi output (MIMO) systems and particularly to a method and apparatus for designing efficient preamble formats with forward/backward compatibility within the 802.11n standard.
2. Description of the Prior Art
Wireless local area network (WLAN) has been a very popular Internet access technology for home, office and public access. The WLAN industry has become so successful that new applications keep invented. Example applications like handset, personal digital assistant (PDA) medium adaptor, and video streaming. The expanding demand returns to drive the further progress of WLAN industry. The Institute of Electrical and Electronics Engineers (IEEE) 802.11n is one of the new WLAN standards to respond to this need.
Multiple-input-multiple-output (MIMO) is prevalently employed in communication equipment and devices. MIMO-OFDM (Orthogonal Frequency Division Multiplexing) is becoming the modulation scheme of the new high speed WLAN standard, the IEEE 802.11n. MIMO-OFDM systems include multiple transmitters and receivers for transmitting and receiving streams of data using OFDM. MIMO-OFDM systems can be denoted as nTmSrR, where n is the number of transmitter antennas, m is the number of data streams and r is the number of receiver antennas. New WLAN standard is expected to support high throughput and extended range through MIMO-OFDM.
WLAN is based on medium sharing technology based on packets instead of direct link. Packets provide excellent framework for sharing. For every transmitted packet, there is a preamble associated with it. A preamble is used to train and initialize receivers before sending data that follows the preamble. To achieve high throughput, the overhead associated with preamble length is critical for high physical (PHY) rate. Efficiency becomes one important criterion for preamble design.
Given the brod scope of the new WLAN standard, many MIMO-OFDM schemes need to work under. the same framework, therefore besides efficiency, interoperability with legacy WLAN devices and forward/backward compatibility within 802.11n modes become another challenge for preamble design.
Previous works on MIMO preamble are showed in proposals from 802.11n (11n) standard subgroups, such as WWiSE and TGnSync group. The major drawback of TGnSync preamble design is lack of efficiency, which will impact throughput especially when packet length is short. While the shortcomings of WWiSE preamble design are lack of flexibility to accommodate different features, such as transmit beamforming, and less well thought on interoperability issue between supported 11n modes.
In light of the foregoing it is desirable to design new preamble formats for efficient performance of MIMO systems which provide interoperability with legacy WLAN devices and forward/backward compatibility with the 802.11n modes. Additionally, the new preamble formats should overcome the aforementioned shortcomings of the current preamble designs.